Tianyi Fan, Owen Brian Toon LASP/ATOC, University of Colorado, Boulder

1. Representation of Sea Salt Aerosol in CAM coupled with a Sectional Aerosol Microphysical Model CARMA Tianyi Fan, Owen Brian Toon LASP/ATOC, University of Colorado, Boulder http://www.tulpule.com/contents/pix/cruises/ccl-ecstasy-dec-01/index.html

2. Introduction • Sea salt aerosols (SSA) scatter solar radiation, modify the properties of clouds, transfer heat and moisture between ocean and atmosphere, and participate in heterogeneous chemistry. • SSA dominates the particulate mass over the remote ocean, with a global emission of 30~3,000 Tg/year [Lewis and Schwartz, 2004]. • The top-of-atmosphere, global annual radiative forcing due to sea salt is estimated between • -1.51 and -5.03 Wm-2 for high and low emission values [IPCC AR3, 2001]. Figure 1. Annual average source strength in kg km-2 hr-1 [IPCC AR3, 2001]

3. Outline • CAM/CARMA Model Description • Production, Wind Particle swelling Dry deposition • Primary results • Problems

4. Model Description Namelist: carma_flag, carma_emission, carma_drydep, carma_vtran, … NCEP Community Aerosol and Radiation Model for Atmospheres Weibull Wind Dry Deposition CAM + + CARMA Production Wet Deposition Particle Swelling Optical Sedimentation Interface module Concentration Nucleation condensational growth/evaporation coagulation [Toon, 1988] 20 bins (0.01 ~ 15 μm) Horizontal: 2o x 2.5o Vertical: 28 layers Optical Depth

5. Sea Salt Production • Difference come within a factor of 2 for radius > 0.5 μm • Significant submicron flux (Clarke2006, Martensson2003) • Gong’s source function applies to 0.02 to 10 μm, doing well for >1 μm. Figure 2. A summary of recent Sea salt source functions [O’Dowd and de Leeuw, 2007]

6. Gong’s Source Function • Number peaks at submicron particles. • Surface area and Mass peaks at > 1 μm. Figure 3. Gong’s source function for number, surface area and mass concentration.

7. NCEP U, V Wind Field Production Weibull Distribution • 10 meter wind from ocean model. It is related to drag. • Production is sensitive to wind speed. • Weibull wind distribution represents the sub-grid-scale characteristics. CAM Data Ocean Model 10 meter wind Neutral Stability Drag Coefficient Friction Velocity Figure 4. Sea salt concentration increases with the introduction of Weibull wind distribution.

8. Particle Swelling • Swelling affects the dry deposition and optical depth calculations. • Gerber’s scheme let particles swell too large at high relative humidity (RH). A constrain to the RH is needed. NCEP QFLX, T Particle Swelling Optical CAM Relative humidity Sedimentation Dry Deposition dry 80% 98%

9. Dry Deposition Scheme CARMA Sedimentation (Vg) CAM Dry Deposition (Vd) Land Model Aerodynamic resistance (ra) Friction velocity Seinfeld and Pandis scheme Figure 5. For large particles, Vd is equal to sedimentation, For small particles, Vd is dominated by mechanisms.

10. Global Distribution: Surface flux and mass concentration 4.e6 2.5e7 4.3e7 5.2e7 7.e7 4.e6 2.5e7 4.3e7 5.2e7 7.e7 Figure 6. Global distribution of surface flux in February and July. Northern hemisphere surface flux is enhanced in February and Southern hemisphere is enhanced in July. Figure 7. Global distribution of concentration at the model bottom level in February and July. Trend is different from surface flux, indicating the effect of sinks.

11. Model Result – Seasonal variation of mass concentration Figure 8. Comparison between the model results and Prospero and Savoie’s observations at locations Cape Point, Mace Head, Bermuda, and Iceland. Model results underestimated the sea salt mass.

12. ①Canonical size distribution: 15% of the mass is outside the range of Gong’s source function • ② Model mass concentration vs. Canonical concentration: • Loss of mass due to overestimated dry deposition of large particles. 85% 15% Canonical mass concentration [Lewis and Schwartz, 2004] Model mass concentration

13. Hoppel’s dry deposition Scheme ① Transport is upward ② Transport is downward Comparison of the deposition velocity by Hoppel et al. [2005] and Slinn and Slinn [1980]

14. Summary • Comparisons with the observations show that sea salt concentrations are underestimated in the model over the global ocean except for the Antarctic. • According to the canonical size distribution, the model results is missing large particles. It is possible due to the size range not described in the source function. • Overestimated dry deposition for large particles may be another reason for low concentrations. An alternative scheme by Hoppel et al. is worth trying to give a lower deposition velocity for large particles.

15. To include Smith’s source function • Smith’s source function measures under very high wind speed (32m/s), providing information on the spume droplet production. • By adding Smith’s source function, we cover the missing mass of the spectrum.